How about, "A US Marine can protect us against gun wielding terrorists on foreign soil, but can't be trusted to stop one gun wielding lunatic at school."?
I wouldn't use "Armed Response" because that terminology is associated by the general public with police duties. I wouldn't use "Workplace Violence" either since it is too general a term and in HR/Safety terms means everything from a shooting to every lesser form of violence. How 'bout, "Being able to defend yourself against an armed lunatic beats cowering in the cubicle."